Can basic

Controller Area Network
q Fundamentals q Physical Layer
• How it all began... • Message Coding
• Overview • Synchronization
• Basic Concepts • Bit Timing / Bit
q Data Link Layer Construction
• Bus Access Arbitration • CAN Bus Lines
• Frame Formats q CAN Implementations
• Error Detection q User Benefits
• Error Handling q Application Examples
• Protocol Versions q Siemens CAN Devices
(Standard /Extended)

Siemens Microelectronics, Inc.
October 98
CANPRES Version 2.0 Slide 1

Page 1

How it all began...

Air
Engine Anti- Lighting Con-
Control Lock dition Power
Brakes Locks

Dash-
board

Airbag
Power
Trans- Active Seats
mission Suspen- Power
Control sion Win-
dows
October 98

•The development of CAN began when more and more electronic
devices were implemented into modern motor vehicles. Examples of
such devices include engine management systems, active suspension,
ABS, gear control, lighting control, air conditioning, airbags and central
locking. All this means more safety and more comfort for the driver and
of course a reduction of fuel consumption and exhaust emissions.

Page 2

How it all began… (cont.)

Air
Brakes Locks

Dash-
board

Airbag
Power
Trans- Active Seats
Control sion Win-
dows
October 98

• improve the behavior of the vehicle even further, it was necessary
To
for the different control systems (and their sensors) to exchange
information. This was usually done by discrete interconnection of the
different systems (i.e. point to point wiring). The requirement for
information exchange has then grown to such an extent that a cable
network with a length of up to several miles and many connectors was
required. This produced growing problems concerning material cost,
production time and reliability.

Page 3

How it all began… (cont.)

Air
Brakes CAN Locks
CAN CAN CAN
CAN
High Speed Dash- Low Speed

CAN

CAN
board
CAN
CAN
Airbag
CAN CAN Power
Seats CAN
Trans- Active
Control sion Win-
dows
October 98

•The solution to this problem was the connection of the control systems
via a serial bus system. This bus had to fulfill some special
requirements due to its usage in a vehicle.
•With the use of CAN, point-to-point wiring is replaced by one serial bus
connecting all control systems. This is accomplished by adding some
CAN-specific hardware to each control unit that provides the "rules" or
the protocol for transmitting and receiving information via the bus.

Page 4

Overview

Specified by Robert Bosch Industrial
CAN GmbH, Germany
Spec
ISO/OSI Automotive
SAE
à Distributed Controls

CAL, CANopen (CiA), 140

7 Application Layer DeviceNet (ODVA), 120
100
SDS (Honeywell)
...

80
CAN

}
60
Nodes

2 Data Link Layer 40
20
CAN 0

1997

1998

1999

2000
1 Physical Layer

October 98

•CAN or Controller Area Network is an advanced serial bus system that
efficiently supports distributed control systems. It was initially developed
for the use in motor vehicles by Robert Bosch GmbH, Germany, in the
late 1980s, also holding the CAN license.
•CAN is internationally standardized by the International Standardization
Organization (ISO) and the Society of Automotive Engineers (SAE).
•The CAN protocol uses the Data Link Layer and the Physical Layer in
the ISO - OSI model. There are also a number of higher level protocols
available for CAN.
•CAN is most widely used in the automotive and industrial market
segments. Typical applications for CAN are motor vehicles, utility
vehicles, and industrial automation. Other applications for CAN are
trains, medical equipment, building automation, household appliances,
and office automation. Due to the high volume production in the
automotive and industrial markets, low cost protocol devices are
available.
•There are about 20 million CAN nodes in use worldwide. By the year
2000 the number of nodes is estimated to be 140 million.
•Examples of vehicle bus systems, other than CAN, are A-BUS from
Volkswagen, VAN or Vehicle Area Network, from Peugeot and Renault,
and J1850 from Chrysler, General Motors and Ford.
•CAN is clearly the leading vehicle bus protocol in Europe.

Page 5

Basic Concepts

n Multimaster n Easy
CAN
Concept connection/
Nodes
disconnection
n Number of of nodes
nodes Node A Node n
not limited by e.g. e.g. n Broadcast/
protocol ABS EMS Multicast
capability
n No node
addressing,
Message
identifier CAN-Bus
specifies (logical)
contents &
priority

October 98

•CAN is a multi-master bus with an open, linear structure with one logic
bus line and equal nodes. The number of nodes is not limited by the
protocol.
• the CAN protocol, the bus nodes do not have a specific address.
In
Instead, the address information is contained in the identifiers of the
transmitted messages, indicating the message content and the priority
of the message.
•The number of nodes may be changed dynamically without disturbing
the communication of the other nodes.
•Multicasting and Broadcasting is supported by CAN.

Page 6

Basic Concepts (cont.)

Node A Node n
n Sophisti-
e.g. ABS e.g. EMS cated Error
Application
Detection /
e.g. Handling
e.g.
Host-Controller 80C166 C167CR
n NRZ Code
or C515C
+ Bit Stuffing
CAN-Controller 81C9x CAN for
Synchroni-
CAN-Transceiver zation

CAN_H UDiff n Bus access
120 120
CAN-Bus Ohm Ohm via
CAN_L EMI 40m / 130 ft CSMA/CD
@ 1 Mbps w/ AMP
October 98

•CAN provides sophisticated error-detection and error handling
mechanisms such as CRC check, and high immunity against
electromagnetic interference. Erroneous messages are automatically
retransmitted. Temporary errors are recovered. Permanent errors are
followed by automatic switch-off of defective nodes. There is
guaranteed system-wide data consistency.
•The CAN protocol uses Non-Return-to-Zero or NRZ bit coding. For
synchronization purposes, Bit Stuffing is used.
•There is a high data transfer rate of 1000 kilobits per second at a
maximum bus length of 40 meters or 130 feet when using a twisted wire
pair which is the most common bus medium used for CAN. Message
length is short with a maximum of 8 data bytes per message and there
is a low latency between transmission request and start of transmission.
•The bus access is handled via the advanced serial communications
protocol Carrier Sense Multiple Access/Collision Detection with Non-
Destructive Arbitration. This means that collision of messages is
avoided by bitwise arbitration without loss of time.

Page 7

Basic Concepts -
CAN Bus Characteristics - Wired-AND

Two logic states
“ = recessive
1”
possible on the bus:
“ = dominant
0”
“ = recessive
1”
“ = dominant
0”

A B C BUS
D D D D
As soon as one node nodes transmits
D D R D
a dominant bit (zero):
D R D D
Bus is in the dominant state.
D R R D
R D D D
R D R D
Only if all nodes transmit
R R D D
recessive bits (ones):
R R R R
Bus is in the recessive state.

October 98

•There are two bus states, called "dominant" and "recessive".
•The bus logic uses a "Wired-AND" mechanism, that is, "dominant bits"
(equivalent to the logic level "Zero") overwrite the "recessive" bits
(equivalent to the logic level "One" ).

Page 8

Basic Concepts -
CAN Bus Characteristics - Wired-AND (cont.)

5V

Bus State: recessive (High)

CAN-Bus
L L L
(single logical
bus line)

R T R T R T
recessive

recessive

recessive
Node A Node B Node C
October 98

•Only if all nodes transmit recessive bits (ones), the Bus is in the
recessive state.

Page 9

Basic Concepts -
CAN Bus Characteristics - Wired-AND (cont.)

5V

Bus State: dominant (Low)

CAN-Bus
H L L
(single logical
bus line)

R T R T R T

recessive

recessive
dominant

October 98

• soon as one node transmits a dominant bit (zero), the bus is in the
As
dominant state.

Page 10

Bus Access and Arbitration: CSMA/CD w/ AMP

Node X
Node A
Node B
Node C

Start Identifier Field
Arbitration Bit
Node A
phase
Node B
Remainder Node C
Transmit CAN Bus
Request
Node B loses Arbitration Node C loses Arbitration
October 98

•The CAN protocol handles bus accesses according to the concept
called “ Carrier Sense Multiple Access with Arbitration on Message
Priority” This arbitration concept avoids collisions of messages whose
.
transmission was started by more than one node simultaneously and
makes sure the most important message is sent first without time loss.
In the picture above you see the trace of the transmit pins of three bus
nodes called A, B and C, and the resulting bus state according to the
wired-AND principle.
• two or more bus nodes start their transmission at the same time after
If
having found the bus to be idle, collision of the messages is avoided by
bitwise arbitration. Each node sends the bits of its message identifier
and monitors the bus level.
• a certain time nodes A and C send a dominant identifier bit. Node B
At
sends a recessive identifier bit but reads back a dominant one. Node B
loses bus arbitration and switches to receive mode. Some bits later
node C loses arbitration against node A. This means that the message
identifier of node A has a lower binary value and therefore a higher
priority than the messages of nodes B and C. In this way, the bus node
with the highest priority message wins arbitration without losing time by
having to repeat the message.
•Nodes B and C automatically try to repeat their transmission once the
bus returns to the idle state. Node B loses against node C, so the
message of node C is transmitted next, followed by node B’ message.
s
• is not permitted for different nodes to send messages with the same
It
identifier as arbitration could fail leading to collisions and errors.
Page 11

Frame Formats - Overview

q Existing Frame Formats:

Data Remote Error
Frame Frame Frame

Inter-
Overload frame
Frame Space

October 98

•These are the existing Frame formats which are now discussed in the
following slides.

Page 12

Frame Formats - Data Frame

October 98

• "Data Frame" is generated by a CAN node when the node wishes to
A
transmit data. The Standard CAN Data Frame is shown above. The frame
begins with a dominant Start Of Frame bit for hard synchronization of all
nodes.
•The Start of Frame bit is followed by the Arbitration Field consisting of 12 bits:
The 11-bit Identifier, which reflects the contents and priority of the message,
and the Remote Transmission Request bit. The Remote transmission request
bit is used to distinguish a Data Frame (RTR = dominant) from a Remote
Frame (RTR = recessive).
•The next field is the Control Field, consisting of 6 bits. The first bit of this field
is called the IDE bit (Identifier Extension) and is at dominant state to specify
that the frame is a Standard Frame. The following bit is reserved and defined
as a dominant bit. The remaining 4 bits of the Control Field are the Data
Length Code (DLC) and specify the number of bytes of data contained in the
message (0 - 8 bytes).
•The data being sent follows in the Data Field which is of the length defined by
the DLC above (0, 8, 16, ...., 56 or 64 bits).
•The Cyclic Redundancy Field (CRC field) follows and is used to detect
possible transmission errors. The CRC Field consists of a 15 bit CRC
sequence, completed by the recessive CRC Delimiter bit.
•The next field is the Acknowledge Field. During the ACK Slot bit the
transmitting node sends out a recessive bit. Any node that has received an
error free frame acknowledges the correct reception of the frame by sending
back a dominant bit (regardless of whether the node is configured to accept
that specific message or not). From this it can be seen that CAN belongs to
the "in-bit-response" group of protocols. The recessive Acknowledge Delimiter
completes the Acknowledge Slot and may not be overwritten by a dominant
bit.
• Page 13
Seven recessive bits (End of Frame) end the Data Frame.

Frame Formats - Remote Frame

October 98

•Generally data transmission is performed on an autonomous basis with
the data source node (e.g. a sensor) sending out a Data Frame. It is
also possible, however, for a destination node to request the data from
the source by sending a Remote Frame.
•There are 2 differences between a Data Frame and a Remote Frame.
Firstly the RTR-bit is transmitted as a dominant bit in the Data Frame
and secondly in the Remote Frame there is no Data Field. In the very
unlikely event of a Data Frame and a Remote Frame with the same
identifier being transmitted at the same time, the Data Frame wins
arbitration due to the dominant RTR bit following the identifier. In this
way, the node that transmitted the Remote Frame receives the desired
data immediately.

Page 14

Frame Formats - Remote Frame (cont.)

q Remote Frame scenario

How hot is the oil ?
Node B
Node A Remote Frame; Identifier 'oil_tmp'
(oil temp.-
115 °C ! sensor)

Data Frame; Identifier 'oil_tmp';
contains desired information ~~~~
115°C
~~~~~

October 98

• a node wishes to request the data from the source, it sends a
If
Remote Frame with an identifier that matches the identifier of the
required Data Frame. The appropriate data source node will then send
a Data Frame as a response to this remote request.

Page 15

Frame Formats - Error Frame

q Active Error Frame: Used for error signaling

October 98

• Error Frame is generated by any node that detects a bus error. The Error
An
Frame consists of 2 fields, an Error Flag field followed by an Error Delimiter
field. The Error Delimiter consists of 8 recessive bits and allows the bus nodes
to restart bus communications cleanly after an error. There are, however, two
forms of Error Flag fields. The form of the Error Flag field depends on the
“error status”of the node that detects the error.
• an “
If error-active” node detects a bus error then the node interrupts
transmission of the current message by generating an “ active error flag” The
.
“active error flag” is composed of six consecutive dominant bits. This bit
sequence actively violates the bit stuffing rule. All other stations recognize the
resulting bit stuffing error and in turn generate Error Frames themselves. The
Error Flag field therefore consists of between six and twelve consecutive
dominant bits (generated by one or more nodes). The Error Delimiter field
completes the Error Frame. After completion of the Error Frame bus activity
returns to normal and the interrupted node attempts to resend the aborted
message.
• an “
If error passive” node detects a bus error then the node transmits an
“passive Error Flag” followed, again, by the Error Delimiter field. The “ passive
Error Flag” consists of six consecutive recessive bits, and therefore the Error
Frame (for an “ error passive” node) consists of 14 recessive bits (i.e. no
dominant bits). From this it follows that, unless the bus error is detected by the
node that is actually transmitting (i.e. is the bus master), the transmission of an
Error Frame by an “ error passive” node will not affect any other node on the
network. If the bus master node generates an “ error passive flag” then this
may cause other nodes to generate error frames due to the resulting bit
stuffing violation.

Page 16

Frame Formats - Overload Frame

q Overload Frame: Used to delay next CAN message

October 98

• Overload Frame has the same format as an “
An active” Error Frame.
An Overload Frame, however can only be generated during Interframe
Space. This is the way then an Overload Frame can be differentiated
from an Error Frame (an Error Frame is sent during the transmission of
a message). The Overload Frame consists of 2 fields, an Overload Flag
followed by an Overload Delimiter. The Overload Flag consists of six
dominant bits followed by Overload Flags generated by other nodes (as
for “active error flag” again giving a maximum of twelve dominant bits).
,
The Overload Delimiter consists of eight recessive bits. An Overload
Frame can be generated by a node if due to internal conditions the
node is not yet able to start reception of the next message. A node may
generate a maximum of 2 sequential Overload Frames to delay the start
of the next message.

Page 17

Frame Formats - Interframe Space

q Interframe Space: Separates a frame (of whatever type) from a
following Data or Remote Frame.

October 98

•Interframe Space separates a preceeding frame (of whatever type)
from a following Data or Remote Frame. Interframe space is composed
of at least 3 recessive bits, these bits are termed the Intermission. This
time is provided to allow nodes time for internal processing before the
start of the next message frame. After the Intermission, for error active
CAN nodes the bus line remains in the recessive state (Bus Idle) until
the next transmission starts.
•The Interframe Space has a slightly different format for error passive
CAN nodes which were the transmitter of the previous message. In this
case, these nodes have to wait another eight recessive bits called
Suspend Transmission before the bus turns into bus idle for them after
Intermission and they are allowed to send again. Due to this mechanism
error active nodes have the chance to transmit their messages before
the error passive nodes are allowed to start a transmission.

Page 18

Error Detection - Overview

q Detected Errors:

CRC Form Stuff
Error Error Error

ACK Bit
Error Error

October 98

•The CAN protocol provides sophisticated error detection mechanisms
discussed in the following slides.

Page 19

Error Detection - Cyclic Redundancy Check

q Calculated and received CRC Checksum must match...

Node A Node B
Calculated Calculated
Idle CRC Checksum: CRC Checksum: Idle
Receive 1234h 1234h Receive
Transmit Transmit
Transmitted Received
CRC Checksum: CRC Checksum:
1234h 1234h match

CAN_H Data Frame

CAN_L
CRC Sequence
October 98

•With the Cyclic Redundancy Check, the transmitter calculates a check
sum for the bit sequence from the start of frame bit until the end of the
Data Field.
•This CRC sequence is transmitted in the CRC Field of the CAN frame.
•The receiving node also calculates the CRC sequence using the same
formula and performs a comparison to the received sequence.

Page 20

Error Detection - Cyclic Redundancy Check (cont.)

q … otherwise Frame was not received correctly (CRC Error)

Node A Node B
Calculated Calculated
Idle CRC Checksum: CRC Checksum: Idle
Receive 1234h 1235h Receive
Transmit Transmit
Transmitted Received
CRC Checksum: CRC Checksum:
1234h 1234h mismatch
CAN_H Error Frame

CAN_L

October 98

• node B detects a mismatch between the calculated and the received
If
CRC sequence , then a CRC error has occurred.
•Node B discards the message and transmits an Error Frame to request
retransmission of the garbled frame.

Page 21

Error Detection - Acknowledge

Ack. Field q A frame
must be
acknow-
Node A Recessive ledged by
Idle at least one
TX other node
Receive
Dominant
Transmit (otherwise
Node B Recessive ACK-Error)
Idle
Receive TX
Dominant
Transmit
Recessive
CAN Bus
Idle
Active Dominant

Ack. Delimiter
Acknowledge Slot
October 98

•With the Acknowledge Check the transmitter checks in the
Acknowledge Field of a message to determine if the Acknowledge Slot,
which is sent out as a recessive bit, contains a dominant bit.
• this is the case, at least one other node, (here node B) has received
If
the frame correctly.
• not, an Acknowledge Error has occured and the message has to be
If
repeated. No Error Frame is generated, though.

Page 22

Error Detection - Frame Check

q No dominant
bits allowed in
CRC Delimiter,
ACK Delimiter,
End of Frame,
1 1 7 3 Intermission
(Form Error)

October 98

•Another error detection mechanism is the Frame Check. If a transmitter
detects a dominant bit in one of the four segments:

CRC Delimiter,
Acknowledge Delimiter,
End of Frame or
Interframe Space

then a Form Error has occurred and an Error Frame is generated. The
message will then be repeated.

Page 23

Error Detection - Bit Monitoring

q A transmitted bit must
be correctly read back
from the CAN bus
(otherwise Bit Error)
q Dominant bits may
11 1 1 overwrite recessive
bits only in the
Arbitration Field and
in the Acknowledge
Slot

October 98

• nodes perform Bit Monitoring: A Bit Error occurs if a transmitter
All

sends a dominant bit but detects a recessive bit on the bus line or,
sends a recessive bit but detects a dominant bit on the bus line.

• Error Frame is generated and the message is repeated.
An

•When a dominant bit is detected instead of a recessive bit, no error
occurs during the Arbitration Field or the Acknowledge Slot because
these fields must be able to be overwritten by a dominant bit in order to
achieve arbitration and acknowledge functionality.

Page 24

Error Detection - Bit Stuffing Check

q 6 consecutive
bits with same
polarity are not
allowed between
Start Of Frame
and CRC
Delimiter
(otherwise Bit
Stuffing Error)

October 98

• six consecutive bits with the same polarity are detected between
If
Start of Frame and the CRC Delimiter, the bit stuffing rule has been
violated.
• stuff error occurs and an Error Frame is generated. The message is
A
then repeated.

Page 25

Error Handling


Error Error Error
reco- reco-
detected gnized gnize
Idle d
Idle Idle
Receive Error Frame Receive Receive
Transmit Transmit Transmit

CAN_H Data Frame

CAN_L

October 98

•Detected errors are made public to all other nodes via Error Frames.
•The transmission of the erroneous message is aborted and the frame
is repeated as soon as possible.

Page 26

Error Handling (cont.)

REC<=127 REC>127
and or TEC>255
TEC<=127 TEC>127

Error Error Bus
Reset active passive off

Re-Initialization only
October 98

•Each CAN node is in one of three error states "error active", "error passive" or
"bus off" according to the value of their internal error counters.
•The error-active state is the usual state after reset. The bus node can then
receive and transmit messages and transmit active Error Frames (made of
dominant bits) without any restrictions. During CAN communication, the error
counters are updated according to quite complex rules. For each error on
reception or transmission, the error counters are incremented by a certain
value. For each successful transaction, the error counters are decremented by
a certain value. The error active state is valid as long as both error counters
are smaller than or equal to 127.
• either the receive or the transmit error counter has reached the value of
If
128, the node switches to the error-passive state. In the error-passive state,
messages can still be received and transmitted, although, after transmission of
a message the node must suspend transmission. It must wait 8 bit times
longer than error-active nodes before it may transmit another message. In
terms of error signaling, only passive Error Frames (made of recessive bits)
may be transmitted by an error-passive node.
• both error counters go below 128 again due to successful bus
If
communication, the node switches back to the error-active state.
•One feature of the CAN protocol is that faulty nodes withdraw from the bus
automatically. The bus-off state is entered if the transmit error counter
exceeds the value of 255. All bus activities are stopped which makes it
temporarily impossible for the station to participate in the bus communication.
During this state, messages can be neither received nor transmitted. To return
to the error active state and to reset the error counter values, the CAN node
has to be reinitialized.

Page 27

Undetected Errors - an example

q 2000 h/year
q 500 kbps
q 25% bus load

q 1 undetected error
every 1000 years

October 98

•To understand the error detection capabilities of CAN, imagine a
vehicle equipped with CAN running 2000 hours per year at a CAN bus
speed of 500 kbps with 25% bus load.
•This will result in 1 undetected error every 1000 years.

Page 28

CAN Protocol Versions

q Two CAN protocol versions available:
• V2.0A (Standard) - 11 bit Message ID’ - 2048 ID’ available
s s

Start of Identifier Control Data Field CRC ACK End
Frame 11 bits Field (0..8 Bytes) Field Field of Frame

• V2.0B (Extended) - 29 bit Message ID’ -
s
more than 536 Million ID’ available
s

Start of Identifier Control Data Field CRC ACK End
Frame 29 bits Field (0..8 Bytes) Field Field of Frame

October 98

•The original CAN specifications (Versions 1.0, 1.2 and 2.0A) specify an
11 bit message identifier. This is known as "Standard CAN".
•Those Data Frames and Remote Frames, which contain an 11-bit
identifier are therefore called Standard Frames.
•With these frames, 211 (=2048) different messages can be identified
(identifiers 0-2047).
•However, the 16 messages with the lowest priority (2032-2047) are
reserved.
•Specification V2.0A has since been updated (to version 2.0B) to
remove this possible message number limitation and meet the SAE
J1939 standard for the use of CAN in trucks.
•Version 2.0B CAN is referred to as "Extended CAN".
•Extended Frames, according to CAN specification V2.0 part B, contain
a 29-bit identifier which allows 229 (over 536 Million) message
identifiers.
•The 29-bit identifier is made up of the 11-bit identifier ("Base ID") and
the 18-bit Extended Identifier ("ID Extension").
•CAN specification Version 2.0B still allows message identifier lengths
of 11 bits to be used.

Page 29

CAN Protocol Versions (cont.)

q Three Types of CAN Modules available (all handle 11 bit ID’
s)
• 2.0A - Considers 29 bit ID as an error
• 2.0B Passive - Ignores 29 bit ID messages
• 2.0B Active - Handles both 11 and 29 bit ID Messages

V2.0B Active
CAN Module

V2.0B Passive
CAN Module

V2.0A CAN
Module

October 98

•There are three different types of CAN modules available.
2.0A - Considers 29 bit ID as an error
2.0B Passive - Ignores 29 bit ID messages
2.0B Active - Handles both 11 and 29 bit ID Messages

Page 30

CAN Protocol Versions (cont.)

Frame with Frame with
11 bit ID 29 bit ID

V2.0B Active
Tx/Rx OK Tx/Rx OK
CAN Module

V2.0B Passive
Tx/Rx OK Tolerated
CAN Module

V2.0A CAN
Tx/Rx OK Bus ERROR
Module

q Care must be taken when mixing protocol versions

October 98

•CAN modules specified after CAN V2.0 part A are only able to transmit
and receive Standard Frames according to the Standard CAN protocol.
•Messages using the 29-bit identifier sent to a Standard CAN module
cause errors.
• a device is specified after CAN V2.0 part B, there's one more
If
distinction. Modules named "V2.0B Passive" can only transmit and
receive Standard Frames but tolerate Extended Frames without
generating Error Frames.
•"V2.0B Active" devices are able to transmit and receive both Standard
and Extended Frames.
•Siemens offers V2.0B Active and V2.0B Passive devices.

Page 31

Message Coding

q Message Coding: NRZ-Code

Signal to Signal to
transmit a “0” transmit a “1”
HIGH HIGH
LOW LOW

“1”
“0”
0 1 0 ...

October 98

•The CAN protocol uses Non-Return-to-Zero or NRZ bit coding. This
means that the signal is constant for one whole bit time and only one
time segment is needed to represent one bit.
•Usually, but not always, a "zero" corresponds to a dominant bit, placing
the bus in the dominant state, and a "one" corresponds to a recessive
bit, placing the bus in the recessive state.

Page 32

Bit Stuffing

q Bit Stuffing ensures sufficient Recessive to Dominant edges
• Stuff Bit inserted after 5 consecutive bits at the same state
• Stuff Bit is inverse of previous bit

1 2 3 4 5 67 8 ... 1 2 3 4 5 67 8 ...

data stream
No. of
consecutive
bits with
Stuff Stuff Stuff
same polarity
Bit Bit Bit
bit stream

1 1 1 2 3 4 5 1 2 3...
12345 12345 1
October 98

•One characteristic of Non-Return-to-Zero code is that the signal
provides no edges that can be used for resynchronization when
transmitting a large number of consecutive bits with the same polarity.
•Therefore Bit stuffing is used to ensure synchronization of all bus
nodes.
•This means that during the transmission of a message, a maximum of
five consecutive bits may have the same polarity.
•Whenever five consecutive bits of the same polarity have been
transmitted, the transmitter will insert one additional bit of the opposite
polarity into the bit stream before transmitting further bits.
•The receiver also checks the number of bits with the same polarity and
removes the stuff bits again from the bit stream. This is called
"destuffing".

Page 33

Bus Synchronization

q Hard Synchronization at Start Of Frame bit

Intermission / SOF ID10 ID9 ID8 ID7
Idle
All nodes synchronize on
leading edge of SOF bit
(Hard Synchronization)

q Re-Synchronization on each Recessive to Dominant edge

Re- Re- Re- Re-
synch synch synch synch
October 98

• contrast to many other field buses, CAN handles message transfers
In
synchronously.
• nodes are synchronized at the beginning of each message with the
All
first falling edge of a frame which belongs to the Start Of Frame bit.
•This is called Hard Synchronization.
• ensure correct sampling up to the last bit, the CAN nodes need to
To
re-synchronize throughout the entire frame. This is done on each
recessive to dominant edge.

Page 34

Bit Construction

q 4 Segments, 8-25 Time Quanta (TQ) per bit time
q Time Quanta generated by programmable divide of Oscillator
q CAN Baud Rate (= 1 / Bit Time) programmed by selection of appropriate TQ
length + appropriate number of TQ per bit

CAN frame

1 Bit Time
1 2 3 4

1 Time
Quantum
October 98

•One CAN bit time (or one high or low pulse of the NRZ code) is
specified as four non-overlapping time segments.
•Each segment is constructed from an integer multiple of the Time
Quantum.
•The Time Quantum or TQ is the smallest discrete timing resolution
used by a CAN node.
• length is generated by a programmable divide of the CAN node's
Its
oscillator frequency.
•There is a minimum of 8 and a maximum of 25 Time Quanta per bit.
•The bit time, and therefore the bit rate, is selected by programming the
width of the Time Quantum and the number of Time Quanta in the
various segments.

Page 35

Synchronization Segment

1 Bit Time
1

Transmit Synchronization Segment
Point

n Next bit is output at start of this segment (Transmit only)
n Bus State change (if any) expected to occur within this
segment by the receivers
n Fixed length of 1 Time Quantum
October 98

•The first segment within a CAN bit is called the Synchronization
Segment and is used to synchronize the various bus nodes.
•On transmission, at the start of this segment, the current bit level is
output.
• there is a bit state change between the previous bit and the current
If
bit, then the bus state change is expected to occur within this segment
by the receiving nodes.
•The length of this segment is always 1 Time Quantum.

Page 36

Propagation Segment

1 Bit Time
2

Propagation Time Segment

n Allows for signal propagation
(across network and through nodes)
n Programmable length (1..8 Time Quanta)

October 98

•The Propagation Time Segment is used to compensate for signal
delays across the network.
•This is necessary to compensate for signal propagation delays on the
bus line and through the electronic interface circuits of the bus nodes.
•This segment may be 1 to 8 Time Quanta long.

Page 37

Phase Buffer Segment 1

1 Bit Time
3

Phase Buffer Sample
Segment 1 Point

n Allows for lengthening of bit during Re-synchronization
n Bus state is sampled at the end of this segment

October 98

•Phase Buffer Segment 1 is used to compensate for edge phase errors.
This segment may be between 1 to 8 Time Quanta long and may be
lengthened during resynchronization.
•The sample point is the point of time at which the bus level is read and
interpreted as the value of the respective bit. Its location is at the end of
Phase Buffer Segment 1 (between the two Phase Buffer Segments).

Page 38

Phase Buffer Segment 2

1 Bit Time
4

Sample Phase Buffer
Point Segment 2

n Allows for shortening of bit during Re-synchronization

October 98

•Phase Buffer Segment 2 is also used to compensate for edge phase
errors. This segment may be shortened during resynchronization.
•Phase Buffer Segment 2 may be between 1 to 8 Time Quanta long, but
the length has to be at least as long as the information processing time
(see below) and may not be more than the length of Phase Buffer
Segment 1.
•The information processing time begins with the sample point and is
reserved for calculation of the subsequent bit level. It is less than or
equal to two Time Quanta long.

Page 39

Bit Lengthening

Needed Sample
tq Point
Phase Phase Next
Sync- Propagation Buffer Buffer Sync-
Segment Time Segment Segment 1 Segment 2 Segment
Transmitter
(slower)

next edge for Re-synchronization delayed by 1 TQ ...
Phase Phase Next
Receiver
(faster)

expected Sample Point
October 98

• a result of resynchronization, Phase Buffer Segment 1 may be
As
lengthened or Phase Buffer Segment 2 may be shortened to
compensate for oscillator tolerances within the different CAN nodes.
• for example, the transmitter oscillator is slower than the receiver
If,
oscillator, the next falling edge used for resynchronization may be
delayed. So Phase Buffer Segment 1 is lengthened...

Page 40

Bit Lengthening (cont.)

Needed Sample
tq Point
Phase Phase Next
Transmitter Segment Time Segment Segment 1 Segment 2 Segment
(slower)

Phase Buffer Segment 1 lengthened by 1 TQ

Phase Phase Next
Receiver
(faster)

New Sample Point
October 98

• in order to adjust the sample point and the end of the bit time.
…

Page 41

Bit Shortening

bit n bit n+1 Needed Sample
Point for bit n+1

Phase Buffer Phase Buffer Sync- Propagation Phase Buffer Phase Buffer Sync-
Segment 1 Segment 2 Segment Time Segment Segment 1 Segment 2 Segment

Transmitter
(faster) next edge for Re-synchronization is 1 TQ too early...

Receiver
(slower) Segment 1 Segment 2 Segment Time Segment Segment 1 Segment 2 Segment

tq expected Sample Point
for bit n+1
October 98

• on the other hand, the transmitter oscillator is faster than the
If,
receiver oscillator, the next falling edge used for resynchronization may
be too early. So Phase Buffer Segment 2 in bit N is shortened...

Page 42

Bit Shortening (cont.)

bit n bit n+1 needed
Sample
Point for bit n+1
Segment 1 Segment 2 Segment Time Segment Segment 1 Segment 2 Segment

Transmitter
(faster) Phase Buffer segment 2 in bit n is shortened by 1 TQ
Phase
Receiver Buffer
Phase Buffer Segment Sync- Propagation Phase Buffer Phase Buffer Sync-
(slower) Segment 1 2 Segment Time Segment Segment 1 Segment 2 Segment

new Sample Point
for bit n+1
October 98

• in order to adjust the sample point for bit N+1 and the end of the bit
…
time

Page 43

Synchronization Jump Width

q Amount by which bit length can be adjusted during Re-synch is defined as
Synchronization Jump Width
• No of TQ by which Phase Buffer Segment 1 can be lengthened
• No of TQ by which Phase Buffer Segment 2 can be shortened

q Programmability of Synchronization Jump Width is mandatory
• Minimum of 1 Tq, maximum of 4 Tq

October 98

•The limit to the amount of lengthening or shortening of the phase buffer
segments is set by the Resynchronization Jump Width.
•The Resynchronization Jump Width may be between 1 and 4 Time
Quanta, but it may not be longer than Phase Buffer Segment 2.

Page 44

Bit Timing

q For ease of programming many CAN Modules combine Propagation Time
Segment and Phase Buffer Segment 1
(i.e. they only use 3 segments)

1 Bit Time

Propagation Phase Buffer
Time Segment Segment 1

+

= Timing Segment 1 Timing
Segment 2
October 98

•For many CAN module implementations, the Propagation Time
Segment and Phase Buffer Segment 1 are combined, for ease of
programming, into one segment often called Timing Segment 1.
•Phase Buffer Segment 2 is then known as Timing Segment 2.

Page 45

Why Program the Sample Position ?

q Early sampling decreases the sensitivity to oscillator tolerances
q Lower cost oscillators (e.g. ceramic resonators) can be used

1 Bit Time
Maximum
Synchronization
Jump Width (4)

Transmit Early
Point Sample
Point

October 98

•Programming of the sample point allows "tuning" of the characteristics
to suit the bus.
•Early sampling allows more Time Quanta in the Phase Buffer Segment
2 so the Synchronization Jump Width can be programmed to its
maximum of 4 Time Quanta.
•This maximum capacity to shorten or lengthen the bit time decreases
the sensitivity to node oscillator tolerances, so that lower cost oscillators
such as ceramic resonators may be used.

Page 46

Why Program the Sample Position ? (cont.)

q Late sampling allows maximum signal propagation time
q Maximum bus length / Poor bus topologies can be handled

1 Bit Time

Time for signal propagation

Transmit Late
Point Sample
Point

October 98

•Late sampling allows more Time Quanta in the Propagation Time
Segment which allows a poorer bus topology and maximum bus length.

Page 47

Relation between Baud Rate and Bus Length

q Up to 1Mbit / sec @40m bus length (130 feet)

1000 Bus lines
500 assumed to be
200
an electrical
medium
Bit Rate 100
(e.g.twisted
[kbps] 50 pair)
20
10
5

0 10 40 100 200 1000 10,000
CAN Bus Length [m]

October 98

•The maximum CAN bus speed is 1 MBaud, which can be achieved
with a bus length of up to 40 meters when using a twisted wire pair.
•For bus lengths longer than 40 meters the bus speed must be reduced.
• 1000 meter bus can still be realised with a 50 KBaud bus speed.
A
•For a bus length above 1000 meters special drivers should be used.

Page 48

CAN Bus Line Characteristics - Wired-AND

5V
Two logic states: CAN-Bus
“ = recessive
1” (single logical
“ = dominant
0” Bus State: dominant bus line)

A B C BUS
D D D D
D D R D
D R D D R T R T R T

recessive

recessive
dominant
D R R D
R D D D
R D R D
R R D D
R R R R

October 98

• CAN is serial bus system with one logical bus line. It has an open,
linear bus structure with equal bus nodes. The number of nodes on the
bus is not restricted by the protocol and may be changed dynamically
without disturbing the communication of other nodes. This allows easy
connection and disconnection of bus nodes, e.g. for addition of system
function, error recovery or bus monitoring.
• The CAN bus line has two logic states: a “ recessive” state and a
“dominant” state. The actual bus state is “ wire-AND” of all node states.
This means, that recessive bits (mostly, but not necessarily equivalent
to the logic level "1") are overwritten by dominant bits (mostly logic level
"0"). As long as no bus node is sending a dominant bit, the bus line is in
the recessive state, but a dominant bit from any bus node generates the
dominant bus state.

Page 49

ISO Physical Layer

q Usual ISO Physical Layer :-
• Bus wires twisted pair, 120R Termination at each end
• 2 wires driven with differential signal (CAN_H, CAN_L)

CAN_Txd Optical Fiber
Optical
CAN_Rxd Transmitter
Receiver
C16xC..
C5xxC..

CAN_Txd
Physical CAN Bus
CAN_Rxd (Differential, e.g Twisted Pair)

CAN_Txd
Differential
CAN_Rxd Bus Driver

October 98

•Therefore, for the CAN bus line, a medium must be chosen that is able
to transmit the two possible bit states “ dominant” and “recessive” One
.
of the most common and cheapest ways is to use a twisted wire pair.
The bus lines are then called "CAN_H" and "CAN_L". The two bus lines
CAN_H and CAN_L are driven by the nodes with a differential signal.
The twisted wire pair is terminated by terminating resistors at each end
of the bus line, typically 120 Ohms.
•But also an optical medium would be possible for CAN. In this case,
the recessive state would be represented by the signal “ light off” the
,
dominant state by the signal “ light on”.

Page 50

CAN and EMI

q CAN is insensitive to electromagnetic interference

V
CAN_H

U diff
CAN_L
t
CAN_H
120 120
Ohm Ohm
CAN_L CAN-Bus
EMI

October 98

•Due to the differential nature of transmission CAN is insensitive to
electromagnetic interference, because both bus lines are affected in the
same way which leaves the differential signal unaffected.
•To reduce the sensitivity against electromagnetic interference even
more, the bus lines can additionally be shielded. This also reduces the
electromagnetic emission of the bus itself, especially at high baudrates.

Page 51

Standardization Issues

CAN
High
Speed
bit rate / kbps CAN
ISO-IS
Low
11898
Speed
ISO-IS
1000
11519-2 Engine
Class C management,
125 Gearbox, ABS
Class B dashboard,
diagnostics
10
Class A body
control
Real-time capability

October 98

•Vehicle bus system applications can be separated in three different
categories according to their real-time capabilities.
•Class A for a low speed bus with bit rates up to 10 kbps, e.g for body
control applications,
•Class B for a low speed bus with bit rates from 10 kbps to 125 kbps,
e.g. for dashboard and diagnostics,
•Class C for a high speed bus with bit rates from 125 kbps to 1 Mbps for
real time applications like engine management, Gearbox, ABS etc.
•For the use of CAN in vehicles two standards have been defined for
the bus interface:
•CAN High Speed according to ISO-IS 11898 for bit rates between 125
kbps and 1 Mbps
•CAN Low Speed according to ISO-IS 11519-2 for bit rates up to 125
kbps

Page 52

Physical Layer according to ISO-IS 11898

CAN
Node 1 Node 32 High
Speed
ISO-IS
11898
… … ...

CAN Bus CAN Bus
Driver Driver
40 m / 130 ft 0.3 m / 1 ft
CAN_H @ 1 Mbps @ 1 Mbps
120 120
Ohm Ohm
CAN_L

October 98

•This is the structure of a Controller Area Network according to ISO-IS
11898. The bus lines may be up to 40 m (130 ft) long at the maximum
speed of 1 Mbaud and are terminated by termination resistors of 120
Ohms. The bus lines may be longer when decreasing the baud rate. Up
to 30 nodes can be connected with CAN drivers according to this
standard. For the connection of more nodes, stronger drivers or
repeaters have to be used. To avoid reflexions the connection from the
bus lines to the nodes should not exceed 0.3 m (1 ft) at 1 Mbps.

Page 53

Bus Levels according to ISO-IS 11898

CAN
High
V Recessive Dominan Recessive Speed
t ISO-IS
5
11898
4 UCAN_H
3.5 V
3 U diff
2.5 V
2
1.5 V
1 UCAN_L

October 98

•These are the bus levels according to ISO-IS 11898. A recessive bit is
represented by both CAN bus lines driven to a level of about 2.5 V so
that the differential voltage between CAN_H and CAN_L is around 0 V.
• dominant bit is represented by CAN_H going to about 3.5 V and
A
CAN_L going to about 1.5 V. This results in a differential voltage for a
dominant bit of about 2V.

Page 54

CAN bus connectors according to CiA-DS 102-1

2: CAN_L 3: 0V (ground)
CiA
bus line 4: reserved DS
102-1

1: reserved 1 2 3 4 5 5: reserved

6: 0V
9: V+ (opt.
(ground)
6 7 8 9 power supply)

7: CAN_H 8: reserved
bus line

October 98

• be able to use CAN as a industrial field bus in an open system the
To
CAN in Automation user’ group CiA created a standard called CiA DS
s
102-1 which is based on the 11898-standard. One important issue in
this standard is the proposal to use a 9-pole SUB-D connector for the
connection of nodes to the CAN bus lines.
•The bus signals CAN_H and CAN_L are available on pins 7 and 2. The
other pins serve as power or ground wires or are reserved for future
extensions of the standard.

Page 55

Typical CAN Implementations

Node A Node B

Application e.g. ABS e.g. EMS

e.g. e.g.
Host-Controller 80C166 C167CR
or C515C

CAN-Controller 81C9x CAN (more
nodes)

CAN-Transceiver

CAN-Bus

October 98

• typical CAN node used to control a certain application consists of
A
different devices.
•The application itself is controlled by a microcontroller, e.g. the
Siemens SAB 80C166.
•To be able to participate in the CAN communication, the
microcontroller has to be connected to a CAN protocol controller, e.g.
the Siemens stand-alone Full-CAN controller 81C90/91.
• meet the requirements of e.g. the ISO 11898 CAN standard, a
To
CAN transceiver chip is used to connect the node to the CAN bus lines.
• more sophisticated way is to use a microcontroller which already has
A
a CAN protocol controller on-chip, e.g. one of the Siemens 8051-
compatible 8-bit microcontrollers from the C500 family like the C505C or
the C515C. For applications which need higher performance, one of the
16-bit C166-family members with integrated CAN module could be
used, e.g. the C164CI or the C167CR. This saves costs as the printed
circuit board space is used more efficiently and the user does not have
to worry about setting up the communication between the
microcontroller and the CAN controller.

Page 56

Basic CAN controller

Status/Control
Registers
L
low high
Host
CPU
CAN CAN
Bus Interface

Bus Protocol
Host CPU load
Controller Transmit
Inter-
Buffer
face Tx Rx
Acceptance Receive
Filtering Buffer(s)

q Use in CANs with very low baudrates and/or very few messages only

October 98

•There is one more CAN characteristic concerning the interface
between the CAN protocol controller and the host CPU, dividing CAN
chips into "Basic-CAN" and "Full-CAN" devices. This has nothing to do
with the used protocol Version though, which makes it possible to use
both Basic-CAN and Full-CAN devices in the same network.
• the Basic-CAN devices, only basic functions concerning the filtering
In
and management of CAN messages are implemented in hardware. A
Basic-CAN controller typically provides one transmit buffer for outgoing
messages and one or two receive buffers for incoming messages. In the
receive path, an acceptance filtering is available which allows that only
certain CAN identifiers are stored in the receive buffer.
•Because there are only two buffers for the reception of messages the
host controller is quite busy reading and storing the incoming messages
before they get overwritten by the following ones which results in a quite
high CPU load. Also the answering of Remote Frames with the
corresponding Data Frame has to be handled by the host controller.
Therefore Basic-CAN devices should only be used at low baudrates and
low bus loads with only a few different messages.

Page 57

Full CAN controller

Status/Control
CAN CAN Registers
Bus
Bus Interface
Protocol
Controller Message
Object 1
Host
Inter- J
face low high

Acceptance Message
Filtering Object 2 CPU load

...
Host CPU
Message
Object n
Receive
Buffer(s)

October 98

•Full-CAN devices provide the whole hardware for convenient
acceptance filtering and message management. For each message to
be transmitted or received these devices contain one so called
message object in which all information regarding the message (e.g.
identifier, data bytes etc.) are stored. During the initialisation of the
device, the host CPU defines which messages are to be sent and which
are to be received. Only if the CAN controller receives a message
whose identifier matches with one of the identifiers of the programmed
(receive-) message objects the message is stored and the host CPU is
informed by interrupt. Another advantage is that incoming Remote
Frames can be answered automatically by the Full-CAN controller with
the corresponding Data Frame. In this way, the CPU load is strongly
reduced compared to the Basic-CAN solution. Using Full CAN devices,
high baudrates and high bus loads with many messages can be
handled.
•Many Full-CAN controller provide a "Basic-CAN-Feature": One of their
message objects behaves like a Basic-CAN Receive Buffer, i.e. it can
be programmed in a way that every message is stored there that does
not match with one of the other message objects. This can be very
helpful in applications where the number of message objects is not
enough to receive all desired messages.

Page 58

Complete CAN Capability –
Products at Each Price-Performance Point

price C167CR
Flash
ROM
C164CI
OTP
ROM
C515C OTP
ROM

C505C
OTP
ROM

81C91
81C90

performance
October 98

User Benefits

q CAN is low cost
• Serial bus with two wires: good price/performance ratio
• Low cost protocol devices available driven by high volume production
in the automotive and industrial markets
q CAN is reliable
• Sophisticated error detection and error handling mechanisms results in
high reliability transmission
• Example: 500 kbit/s, 25% bus load, 2000 hours per year:
One undetected error every 1000 years
• Erroneous messages are detected and repeated
• Every bus node is informed about an error
• High immunity to Electromagnetic Interference

October 98

1

Can basic

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